Liquid heater

ABSTRACT

A firetube is mounted on a tube sheet for immersion in liquids to be heated in a vessel, or container. A web plate is connected between the tube legs and the tube sheet. A restraining structure is attached near the end of the tube.

1 =1 ,7 v I Bite Staes 1 atent 1 1 11 1 3,851,625 Holt Dec. 3, 1974 [54] LlQUllD HEATER l,908,l25 5/1933 Davidson 122/135 6 2 1119 H ...l634. [75] Inventor: Flecher Tulsa, Okla- 51 1351233 1/1953 5315i: 126/3435 73 Assigneez Combustion Engineering, Inc. N 2,941,525 6/1960 Harshfield 126/360 X York N Y 3,028,843 4/1962 Carlson et a1... 122/149 3,495,575 2/1970 Mansfield 122/149 [22] Filed: Oct. 11, 1973 21 No: Primary Examiner-Kenneth Sprague Attorney, Agent, or Firm-Arthur L. Wade [52] U.S. Cl. 122/136 R, 122/149, 126/360 R 51 1111.01. F22b 7/00 [57] Y ABSTRACT [58] Fi ld f S h 122/135 R 136 R 136 C A firetube is mounted on a tube sheet for immersion 122 149; 12 3435 3 0; 1 5 7 1 2 in liquids to be heated in a vessel, or container. A web plate is connected between the tube legs and the tube [56] R f r Cit d sheet. A restraining structure is attached near the end UNITED STATES PATENTS of the tube- 1,45I,l26 4/1923 Sundstrand 126/360 3 Claims, 3 Drawing Figures LIQUID HEATER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to control of destruc- 2. Description of the Prior Art For many years, there have been a series of mysterious failures of firetubes in oil field process equipment. It has long been the practice to immerse a hollow tube in produced liquids and fire oil or gas burners into the tube to heat the external liquids. Both the thermal and buoyancy stress on the tubes has caused cyclic displacement of the tubes as they are mounted from their ends on their vessel walls.

The tubes have been fabricated with welded seams. Failures of these seams have resulted in leakage of the heated liquids into the tubes. The reasons for most of these failures are readily traceable. Poor welding has been the usual reason. Scale has been deposited on the tube surface. Localized areas of the tube surface have been overheated and failed. Leakage of the heated liquids has resulted.

However, the tube sheet on which each tube has been mounted has failed in a manner which has not been fully understood. Cracks have appeared through the tube sheet as a wall. With the firetube having a U, or hairpin, shape, mounted on the tube sheet by the ends of its two legs, the cracks have appeared in the tube sheet above the lower of the two legs.

To eliminate the tube sheet failures, different configurations of tube sheet and tube have been tried. As an example, the thickness of the tube sheet has been increased to withstand more stress. As another example, the shape of the legs of the tube has been altered. However, none of these changes have gained significant improvement, particularly in those tubes extending for lengths of feet, and more, from their tube sheet. Cracks have continued to develop within one year, and often within weeks, of the tubes being placed in service. Workers in the art generally agree that the forces of buoyancy and thermal stress are severe and one, or some combination, of these forces produces the failure. However, for years, no one has produced a solution to the problem.

SUMMARY OF THE INVENTION A principal object of the invention is to distribute the stress placed on a firetube heating liquids.

Another object is to counteract, or reduce, stress on the tube sheet mount of a firetube by a stress-bearing structure attached to the firetube.

Another object is to restrain movement of a firetube so stress will not result in failure of the tube, or its tube sheet mount. Y

The present invention is embodied in a structural member attached between the legs of a U-shaped, or hairpin-shaped, firetube and extending substantially the length of the legs of the tube. In more specific form, the structural member is a plate, or beam, of substantial thickness and is welded to the walls of the legs and the tube sheet through which the ends of the legs are connected. The plate, or beam, may then be described as a web which bridges the space normally lying between the parallel legs of the tube. Such Web then adds to the strength of the firetube which, as an over-all unit, may be characterized as a cantilevered beam.

The present invention also includes the addition of hold-down structure, or restraint, attached to the tube a significant distance from the tube sheet mount of the tube. This mechanical hold-down structure limits the movement of the tube in a plane normal to the plane of the tube sheet mount and, together with the web, prevents the stress on the tube from causing failure of the tube sheet.

Other objects, advantages and features of this invention will become apparent to one skilled in the art upon consideration of the written specification, appended claims, and attached drawings, wherein;

FIG. 1 is a side elevation of a firetube as mounted in a vessel to heat liquids and embodying the present invention;

FIG. 2 is a section of FIG. I along lines 2-2; and

FIG. 3 is a side elevation of .the firetube of FIG. 1 with a second form of tie-down.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, a U-shaped firetube l is shown as mounted on tube sheet 2. Tube sheet 2 is flanged at 3 for connection to a vessel wall 4 which has a matching flange 5 about an opening 6.

The tube sheet is a mount for the firetube in that parallel legs 7 and 8 are extended through openings in tube sheet 2 and connected to the tube sheet. The usual connection between tubes and sheets is welding. Keep in mind that the tube sheet must be sized to support the firetube as a beam, cantilevered from the attachment to the tube sheet.

To orient observation of the structure, a burner 9 is shown mounted on the lower leg 8 of the firetube l. A stack 10 is shown mounted on the upper leg 7. The vessel, or container, is to be considered full of liquid; at least full enough to immerse tube 1. Therefore, the hollow tube is subjected to the tremendous force of buoyancy acting upward on tube 1. The result is tremendous stress on the tube sheet 2. Perhaps this upward force is the largest factor in the picture. Perhaps this is the basic, or dominant, cause of past failures of the tube sheet in the area 11, just above the lower leg 8. However, it makes little difference how the stresses are analyzed; they occur in some combination and are successfully controlled by the present invention to prevent cracking of the tube sheet 2 at area 11.

The basic structural improvement in which the invention is embodied is a beam, or plate, 12. Beam 12 spans the distance between the upper surface of leg 8 and the lower surface of leg 7. Therefore, it can be logically termed a web. This web extends from, or near, tube sheet 2, substantially the entire length of the legs 7,8.

The web may, or may not, be connected to the tube sheet. The web is connected to the tube surfaces and the tube sheet. This connection is usually welding.

The welding which connects the tube legs 7 and 8 with the web-plate 12 can be seen to some advantage in FIG. 2. In FIG. 1, the structural addition to the firetube can be seen as extending from its connection to tube sheet 2 almost the complete length of the legs 7 and 8. An opening remains at 13. Although not shown in FIG. 2, an opening may be provided between the web-plate l2 and the tube sheet 2. A tie-down for the end of the firetube 1 can utilize the opening 13 as will be apparent in FIG. 3.

In FIG. 1, the end of the tube 1 is stabilized by a tie-down structure which is simply a mechanical link for the tube and to a reference point. By reference point, is meant a structure which is fixed in relation to the container in which the tube 1 is mounted. Beam 14 is the embodiment of such reference point.

One structural link, or tie-down, between the end of tube 1 and beam 14 is shown in FIG. 1. An extension 15 is mounted on the end of tube 1. As a male member, extension 15 fits into a socket 16 which is attached to beam 14. The two coupling members are aligned and joined as tube 1 is inserted through opening 6 in vessel wall 4. The movement of the end of tube 1 is limited by this connection. Any force brought to bear upon tube 1 will be successfully limited by the connection.

FIG. 3 is established only to disclose an alternate form of tie-down. The tie-down of FIG. 1 is somewhat elaborate and perhaps quite costly. FIG. 3 shows how beam 17 can serve as a reference point and a simple U- bolt 18 links tube 1 to beam 17. A manway can be provided in the wall of the vessel through which this link can be installed.

The invention as disclosed was reduced to practice and tested. The pertinent sections of the report on the test are suitable to restate the objectives of the invention and describe the function of the embodying structure in achieving the objectives. Therefore, the concluding section of the report is quoted as follows:

It is believed that this type of construction will reduce firetube failures due to fatigue failure. There are several stresses that exist in the firetube during operation. They are hoop stresses, thermal stresses, torque stresses due to buoyancy of the firetube, and stresses due to different elongations of the two tubes. Since the normal operation is cyclic in nature, there are probably stress reversals set up in the firetube. The combined stresses acting on the firetube are difiicult to determine. Imperfections in welding fabrications, such as undercuts, slag inclusions, etc. only tend to reduce the life of firetube where fatigue failures occur.

The thought of adding a web to connect the bottom leg of firetube to the top leg of the firetube was for the following purposes:

1. To help provide a conductive path from the bottom tube to the top tube to reduce the temperature difference of the two legs and therefore reduce the elongation difference of the two tubes.

- 2. To produce a more rigid construction of the firetube by connecting the bottom leg to the top leg. This will result in a construction that has more section modulus and will be stiffer. This will prevent the tubes from trying to bow when there is a temperature difference in the two. This bowing could set up stress reversals which reduce the life of tube.

3. To provide a means to distribute the forces due to elongation differences, buoyancy, etc. over a larger area, thereby reducing the stresses in any one area.

. Without the web tying the two tubes together, the existing forces must be reacted in both the tube sheet and the 180 return bend. Since the area of the attachment of tube sheet or return bend is essentially fixed, the stress is the unbalanced force divided by the attached area (S F/a). By increasing the area of attachment all along the fire-tube, the stress at the tube sheet and return must be reduced. In essence, the elongation of both tubes is the same since they are attached to each other. Stresses due to temperature differences are reduced because there is a conductivepath for the two tubes. Since the fabrication of the fire tube is that of rigid construction, there will probably be less stress reversals i.e., switching from a stress in tension to a stress in compression. Without stress reversals, there can be no fatigue failure.

CONCLUSION It is realized that the problems solved by this invention, and the invention itself, appear simple. On the other hand, the stresses produced are not simple to analyze. Further, for years the problem was attacked from many viewpoints without success.

The report on the test of the reduction to practice gives a good, grassroots, picture of the problems and practical approach to their solution. The embodiment of the invention which solves the problem looks simple, and perhaps it is. But the invention solves a problem of long standing in the art and has saved atrernendous amount of material, labor, and, of course, money.

From the foregoing, it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and inherent to the method and apparatus. I

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the invention.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted in an illustrative and not in a limiting sense.

The invention, having been described, what is claimed is:

l. A firetube of U-shape extended horizontally, including,

a tube sheet through which the ends of the legs of the U-shaped tube are mounted and to which the legs are welded, and

a flat plate extended between the parallel legs of the tube for a substantial length of the space between the legs and attached to the walls of the legs by welding along the length of the plate.

2. The firetube of claim 1, including,

a welded connection between the flat plate and that portion of the tube sheet between the legs.

3. The fir'etube of claim 1, including,

a structural link between the tube at a substantial distance from the tube sheet mount and a stable point of reference which will restrict the movement of the firetube in the plane normal to that of the plane of the tube sheet. 

1. A firetube of U-shape extended horizontally, including, a tube sheet through which the ends of the legs of the U-shaped tube are mounted and to which the legs are welded, and a flat plate extended between the parallel legs of the tube for a substantial length of the space between the legs and attached to the walls of the legs by welding along the length of the plate.
 2. The firetube of claim 1, including, a welded connection between the flat plate and that portion of the tube sheet between the legs.
 3. The firetube of claim 1, including, a structural link between the tube at a substantial distance from the tube sheet mount and a stable point of reference which will restrict the movement of the firetube in the plane normal to that of the plane of the tube sheet. 